Flat-panel displays have an array of pixels distributed in a display viewing area over a display substrate. The pixels are electrically controlled using matrix addressing with the intersection of row and column wires on the substrate defining pixel locations and from which rows of pixels in the array are sequentially provided with control signals. Passive-matrix control relies on row and column controllers external to the display viewing area to sequentially enable rows of pixels to emit light, so that only one row of pixels at a time emits light. Active-matrix control relies on local storage and control provided in the display viewing area for each pixel, for example with a storage capacitor and driving transistor and as disclosed in U.S. Pat. No. 9,117,940. Data is provided from an external column controller to selected rows of pixels in sequence and the rows are sequentially selected with a row controller. The pixels in each selected row receive data on the column wires and store the data locally in the pixel. Once the data is received and stored, it is displayed at each pixel by the control circuitry in the pixel by providing power to the pixel light controllers, for example electrodes controlling a liquid crystal (in the case of a liquid crystal display) or an organic light-emitting diode (in the case of an OLED display).
Resolution is an important performance attribute for displays and is calculated as the number of pixels, or light-emitters, per linear metric in one or both dimensions of the flat-panel display. For example, a display can have a resolution of 250 pixels per inch (ppi) or dots per inch (dpi). Commercially available cell phones have a resolution of 326 ppi. Typically, the resolution of a display is the same in both orthogonal (x, y) display dimensions over the surface of the display, but it is not necessarily so. In general, a greater resolution is preferred because more information can be displayed in a more pleasing way on a higher-resolution display.
Increased resolution displays are made using manufacturing processes having reduced tolerances and reduced component sizes, as well as requiring increased operating power and are typically achieved in an active-matrix display by reducing the size of the control circuitry, for example including the wires, light-control electrodes, transistors, and capacitors in a pixel circuit. However, this size reduction increases manufacturing costs and there are limits to the reduction in size of the circuits, especially in flat-panel displays that rely on layers of amorphous silicon or polysilicon to form the circuits.
Active-matrix liquid crystal displays (LCD) circuits generally require only one transistor to control the liquid crystals and therefore do not use much area on a display substrate. Organic light-emitting diode (OLED) displays typically require larger and more complex control circuits than LCDs. The larger circuits increase the size of the OLED pixels for bottom-emitting displays that emit light through the substrate, since the substrate area must be shared between the pixel circuits and the OLED light-emitters. Top-emitting OLED displays locate the OLED light emitters in a layer over the pixel circuits and are not as limited in resolution by the size of the pixel circuit. In both cases, pixels are preferably as large as possible to increase display brightness and lifetime of the OLEDs. For both of these technologies, control circuits are implemented in thin-film layers of silicon provided over the flat-panel display substrate; as the size of the display substrate increases so does the cost of providing high-quality silicon over the substrate. Inorganic light-emitting diode (iLED) displays have many advantages, such as efficiency, color purity, and lifetime, and are found today in digital signage and large-format displays, for example in sporting venues. The iLEDs in these displays are mounted in a display frame, for example in an array of tiles, and controlled by circuitry external to the display frame. It is difficult, therefore to construct a high-resolution iLED display. Existing iLED displays often have pixel pitches of approximately 1 mm (25 ppi), a relatively low resolution, especially compared to flat-panel displays.
One approach to providing high-performance electronic circuits distributed over a large display substrate is described in “AMOLED Displays using Transfer-Printed Integrated Circuits” published in the Proceedings of the 2009 Society for Information Display International Symposium Jun. 2-5, 2009, in San Antonio Tex., US, vol. 40, Book 2, ISSN 0009-0966X, paper 63.2 p. 947. In this approach, small integrated circuits are formed over a buried oxide layer on the process side of a crystalline wafer. The small integrated circuits, or chiplets, are released from the wafer by etching the buried oxide layer formed beneath the circuits. A PDMS stamp is pressed against the wafer and the process side of the chiplets is adhered to the stamp. The chiplets are pressed against a destination substrate or backplane coated with an adhesive and thereby adhered to the destination substrate. The adhesive is subsequently cured, electrodes are formed, and OLED material layers evaporated over the substrate to form light-emitting pixels.
In another example, U.S. Pat. No. 8,722,458 entitled Optical Systems Fabricated by Printing-Based Assembly teaches transferring light-emitting, light-sensing, or light-collecting semiconductor elements from a wafer substrate to a destination substrate or backplane. U.S. Pat. No. 7,972,875 entitled Optical Systems Fabricated by Printing Based Assembly discloses assembling printable semiconductor elements on a substrate via contact printing. U.S. Patent Publication No. 2016/0351539 entitled Inorganic-Light-Emitter displays with Integrated Black Matrix describes an inorganic light-emitting display with micro-transfer printed light-emitting diodes and pixel controllers distributed over a flat-panel display substrate with an integrated black matrix. Certain embodiments of such approaches provide an LED display with improved optical performance but there remains a need for inorganic LED displays with increased resolution and pixel structures that facilitate such increased resolution.